The present invention relates to a coated cemented carbide cutting tool insert particularly useful for toughness demanding wet and dry machining, preferably milling, of low and medium alloyed steels and stainless steels, with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces under unstable conditions.
When machining low and medium alloyed steels and stainless steels with cemented carbide tools, the cutting edge is subjected to wear according to different mechanisms, such as chemical wear, abrasive wear, adhesive wear and to edge chipping caused by cracks formed along the cutting edge, the so called comb cracks. Under severe cutting conditions, bulk and edge line breakages commonly occur.
Different work piece materials and cutting conditions require different properties of the cutting insert. For example, when cutting steel components with raw surface zones or cutting under other difficult conditions, a coated cemented carbide insert must be based on a tough carbide substrate and have a coating with excellent adhesion. When machining low alloyed steels and stainless steels, the adhesive wear is generally the dominating wear type. Here, generally thin (1-3 μm) CVD- or PVD-coatings have to be used.
Measures can be taken to improve or optimize cutting performance with respect to a specific wear type. However, very often such measures will have a negative effect on other wear properties.
The influence of some possible measures is given below:
1.) Comb crack formation can be reduced by lowering the binder phase content. However, low binder content will lower the toughness properties of the cutting inserts which is far from desirable.
2.) Improved abrasive wear can be obtained by increasing the coating thickness. However, thick coatings increase the risk for flaking and will also lower the resistance to adhesive wear.
3.) Machining at high cutting speeds and at other conditions leading to high cutting edge temperatures require a cemented carbide with higher amounts of cubic carbides (solid solution of WC—TiC—TaC—NbC), but such carbides will promote comb crack formation.
4.) Improved toughness can be obtained by increasing the cobalt binder content. However, high cobalt content decreases the resistance to plastic deformation.
So far it has been very difficult to improve all tool properties simultaneously. Commercial cemented carbide grades have therefore been optimized with respect to one or a few of the mentioned wear types and hence to specific cutting application area.
U.S. Pat. No. 6,062,776 discloses a coated cutting tool insert particularly useful for milling in low and medium alloyed steel with or without raw surface zones during wet or dry conditions. The insert is characterized by WC—Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase, a coating including an innermost layer of TiCxNyOz with columnar grains and a layer of κ-Al2O3 with a top layer of TiN.
U.S. Pat. No. 6,406,224 discloses a coated cutting tool insert also particularly useful for milling of alloyed steel with or without abrasive surface zones at high cutting speeds. The coated cutting tool insert consists of a cemented carbide body with a composition of 7.1-7.9 wt % Co, 0.2-1.8 wt % cubic carbides of the metals Ta, Nb and Ti and balance WC. The insert is coated with an innermost layer of TiCxNyOz with columnar grains and a layer of κ-Al2O3 with a top layer of TiN.
EP-A-736615 discloses a coated cutting insert particularly useful for dry milling of grey cast iron. The insert is characterized by having a straight WC—Co cemented carbide substrate and a coating consisting of a layer of TiCxNyOz with columnar grains and a top layer of fine grained textured α-Al2O3.
US 2006/0204757 A discloses a coated cutting tool insert suitable for machining of metals by turning, milling, drilling or by similar chip forming machining methods. The tool insert is particularly useful for interrupted toughness demanding cutting operations.
U.S. Pat. No. 6,200,671 discloses a coated turning insert particularly useful for turning in stainless steel. The insert is characterised by a WC—Co-based cemented carbide substrate having a highly W-alloyed Co-binder phase and a coating including an innermost layer of TiCxNyOz with columnar grains and a top layer of TiN and an inner layer of fine grained κ-Al2O3.